1. Field of the Invention
The present invention relates to endosteal fixation of a ligament by device insertion and, more specifically, to methods and devices for bicortical tibial fixation of anterior cruciate ligament grafts.
2. Description of the Related Art
When a ligament becomes detached from a bone, surgery usually is required to reconstruct the ligament. Often, a substitute ligament or graft is secured into bone tunnels to facilitate incorporation and permanent attachment.
Various methods of graft attachment are known, including the use of interference screws to secure the graft against the walls of a tunnel drilled through the tibia and a socket formed in the femur. A strong graft attachment is obtained by using a metal interference screw to wedge a graft bone block to the wall of a graft tunnel formed through the bone, as disclosed in U.S. Pat. No. 5,211,647 to Schmieding. If a bioabsorbable interference screw is used, the graft can be wedged directly against the bone by the screw, without a bone component.
In either case, the graft usually is secured as close as possible to the normal ligament origin and insertion site, which are at the top of the tibial tunnel (the tibial plateau) and the entrance to the femoral socket in ACL reconstructions. The portion of the graft extending out the bottom of the tibia is ordinarily secured to the outside of the bone with a staple or using screw/washer fixation.
The above-described secondary fixation of the graft to the exterior surface of the tibia is disadvantageous in that it is subject to abrasion from external elements, and is generally less secure than internal fixation. Accordingly, a graft fixation technique is needed which provides increased fixation strength of the graft in the tibial tunnel, and improved healing of the tibial tunnel and associated tissue.
The present invention overcomes the disadvantages of the prior art and achieves the foregoing objectives by providing apparatus and methods for bicortical fixation of ligament grafts, whereby the graft is fixed at two cortical locations (xe2x80x9cbicorticalxe2x80x9d) within the tibial tunnel using a pair of fixation devices. The invention advantageously improves fixation strength, and also minimizes the likelihood of damage to the graft and the bone tunnel during and after fixation, such as by preventing widening of the bone tunnel by graft motion. In addition, as described below, bicortical fixation improves the healing environment of the ligament graft.
The fixation strength of the graft is advantageously increased by engaging the graft against the denser, cortical bone at the ends of the tunnel. The fixation method and devices of the present invention preferably are designed to match the anatomy of the tibial tunnel, and to provide fixation at the original insertion point of the ligament. The fixation devices also are designed to minimize graft abrasion, while maximizing fixation strength.
Further, the preferred fixation methods and devices advantageously restrict blood loss from the fixation site to improve healing and graft incorporation. The preferred fixation modes advantageously plug both ends of the bone tunnel, and leave the internal bone tunnel cavity unobstructed between the plugged ends. Accordingly, the bone tunnel cavity, through which the graft passes, is allowed to fill with serous fluids to promote faster healing and enhance graft incorporation within the tunnel.
Various modes of fixation can be used in the present invention, including, for example, interference screws, wedges, expanding devices, and adhesives. Preferred alterative devices are those that securely engage the cortical wall of the tunnel, and preferably include threads, ridges, and/or other enhancements to maximize bone fixation.
Preferred methods and devices disclosed herein utilize interference screw fixation, although any other type of fixation device capable of being secured bicortically also could be used. Further, identical modes of fixation need not be used at both ends of the tunnel. Preferably, the mode of fixation also will at least substantially occlude both ends of the bone tunnel, resulting in the further advantage of an improved healing environment within the tunnel, as described further below.
According to a preferred embodiment using interference screw fixation, the interference screws used in the present invention preferably have a hex socket for receiving a hex-head screwdriver. The hex socket extends substantially the length of the screw to optimize the distribution of insertion torque along the length of the screws. In order to maintain wall thickness, the hex socket is tapered in correspondence with the tapered outer profile of the device. The taper also permits easy insertion of the hex driver (also tapered) into the fixation screw. A cannulated hex-head screwdriver is used for guide pin insertion methods.
The interference screws preferably are fully-threaded to maximize fixation strength within the tunnel. Preferably, the proximal screw (i.e., the screw closest to the joint) has a smooth, rounded tip profile so as to minimize abrasion with the graft. The distal screw (i.e., the screw farthest from the joint) has an angled back end so that it can be oriented substantially flush with the outer surface of the bone (e.g., the tibia) into which the screw has been installed. These and other features for minimizing graft abrasion and maximizing graft fixation also apply to the other modes of bicortical fixation envisioned by the present invention.
The fixation devices of the present invention, preferably interference screws, optimally are formed of a bioabsorbable material. Bioabsorbable materials known to those of skill in the art include poly-(L-lactic acid) (PLA), poly-(D,L-lactide), and poly glycolic acid (PGA), for example. Other bioabsorbable, non-metallic materials, especially those tailored for hardness, tensile strength, and compressive strength may be utilized. Other known biocompatible materials which could be used include plastics, titanium, titanium alloys, allograft bone, and inert bone substitute materials.
In the preferred method of ACL reconstruction of the present invention, the graft (preferably a hamstring tendon autograft or allograft) is secured femorally preferably by interference screw fixation in a socket formed through the tibial tunnel, as described, for example, in U.S. Pat. No. 5,320,626, the disclosure of which is incorporated herein by reference. The preferred femoral interference screw is inserted into the femoral socket, and has a rounded back end to prevent tissue damage after insertion. Other forms of femoral fixation also could be used.
Bicortical tibial fixation is provided by delivering the proximal fixation device to the inner opening of the tibial tunnel and installing the device to secure the ligament graft at the anatomical position on the tibial plateau. The distal device is delivered and installed to secure the graft within the tibial tunnel at the outer end of the tunnel. Prior to device insertion, the tunnel may be pre-tapped and/or dilated to enhance interference fixation.
A guide pin preferably is employed as necessary to guide the femoral interference screw and the proximal tibial interference screw during delivery and installation. For this reason, these two devices preferably are fully cannulated. The distal tibial screw, on the other hand, preferably is non-cannulated, to prevent blood from flowing from the tibial tunnel and into the surrounding tissue.
Femoral graft insertion and fixation can be achieved by various methods and devices known in the art, including the transverse, intraosseous pin and technique disclosed in allowed U.S. patent application Ser. No. 09/015,618, filed Jan. 29, 1998, or in U.S. Pat. No. 5,601,562, the disclosures of which are incorporated herein by reference.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.